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  <div class="question_difficulty">
   难度：Easy
  </div>
  <div>
   <h1 class="question_title">
    800. Letter Case Permutation
   </h1>
   <p>
    Given a string S, we can transform every letter individually&nbsp;to be lowercase or uppercase to create another string.&nbsp; Return a list of all possible strings we could create.
   </p>
   <pre>
<strong>Examples:</strong>
<strong>Input:</strong> S = "a1b2"
<strong>Output:</strong> ["a1b2", "a1B2", "A1b2", "A1B2"]

<strong>Input:</strong> S = "3z4"
<strong>Output:</strong> ["3z4", "3Z4"]

<strong>Input:</strong> S = "12345"
<strong>Output:</strong> ["12345"]
</pre>
   <p>
    <strong>
     Note:
    </strong>
   </p>
   <ul>
    <li>
     <code>
      S
     </code>
     will be a string with length between
     <code>
      1
     </code>
     and
     <code>
      12
     </code>
     .
    </li>
    <li>
     <code>
      S
     </code>
     will consist only of letters or digits.
    </li>
   </ul>
  </div>
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   <h1 class="question_title">
    800. 字母大小写全排列
   </h1>
   <p>
    给定一个字符串
    <code>
     S
    </code>
    ，通过将字符串
    <code>
     S
    </code>
    中的每个字母转变大小写，我们可以获得一个新的字符串。返回所有可能得到的字符串集合。
   </p>
   <pre>
<strong>示例:</strong>
<strong>输入:</strong> S = "a1b2"
<strong>输出:</strong> ["a1b2", "a1B2", "A1b2", "A1B2"]

<strong>输入:</strong> S = "3z4"
<strong>输出:</strong> ["3z4", "3Z4"]

<strong>输入:</strong> S = "12345"
<strong>输出:</strong> ["12345"]
</pre>
   <p>
    <strong>
     注意：
    </strong>
   </p>
   <ul>
    <li>
     <code>
      S
     </code>
     &nbsp;的长度不超过
     <code>
      12
     </code>
     。
    </li>
    <li>
     <code>
      S
     </code>
     &nbsp;仅由数字和字母组成。
    </li>
   </ul>
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